Question

To start out here is a simplified version of the tables involved.

tbl_map has approx 4,000,000 rows, tbl_1 has approx 120 rows, tbl_2 contains approx 5,000,000 rows. I know the data shouldn’t be consider that large given that Google, Yahoo!, etc use much larger datasets. So I’m just assuming that I’m missing something.

    CREATE TABLE `tbl_map` (
      `id` bigint(20) NOT NULL AUTO_INCREMENT,
      `tbl_1_id` bigint(20) DEFAULT '-1',
      `tbl_2_id` bigint(20) DEFAULT '-1',
      `rating` decimal(3,3) DEFAULT NULL,
      PRIMARY KEY (`id`),
      KEY `tbl_1_id` (`tbl_1_id`),
      KEY `tbl_2_id` (`tbl_2_id`)
    ) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

    CREATE TABLE `tbl_1` (
      `id` bigint(20) NOT NULL AUTO_INCREMENT,
      PRIMARY KEY (`id`)
   ) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

    CREATE TABLE `tbl_2` (
      `id` bigint(20) NOT NULL AUTO_INCREMENT,
      `data` varchar(255) NOT NULL DEFAULT '',
      PRIMARY KEY (`id`),
    ) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

The Query in interest: also, instead of ORDER BY RAND(), ORDERY BY t.id DESC. The query is taking as much as 5~10 seconds and causes a considerable wait when users view this page.

EXPLAIN SELECT t.data, t.id , tm.rating
FROM tbl_2 AS t
JOIN tbl_map AS tm 
ON t.id = tm.tbl_2_id
WHERE tm.tbl_1_id =94
AND tm.rating IS NOT NULL
ORDER BY t.id DESC
LIMIT 200 

1   SIMPLE  tm  ref     tbl_1_id, tbl_2_id  tbl_1_id    9   const   703438  Using where; Using temporary; Using filesort
1   SIMPLE  t   eq_ref  PRIMARY     PRIMARY     8   tm.tbl_2_id     1 

I would just liked to speed up the query, ensure that I have proper indexes, etc.
I appreciate any advice from DB Gurus out there! Thanks.

Answer 1

SUGGESTION : Index the table as follows:

ALTER TABLE tbl_map ADD INDEX (tbl_1_id,rating,tbl_2_id);